Multiphase screw pump

ABSTRACT

A twin screw pump includes a pair of rotors disposed inside a casing having an inlet and an outlet. Each rotor includes a set of threads disposed on a portion of an outer surface of a shaft. A first bearing is coupled at a first end of the shaft and a second bearing is coupled at a second end of the shaft. The first and second bearings are not separated by one or more seals. The first and second bearings are configured for being lubricated by the liquid medium of the process fluid when the twin screw pump is in an operational mode.

BACKGROUND

The embodiments disclosed herein relate generally to a screw pump, and more particularly to a rotor assembly of a multi-phase twin screw pump.

Twin screw pumps are rotary, positive displacement pumps that use two screws to transfer high or low viscosity fluids or fluid mixtures along an axis. Typically twin screw pumps have two intermeshing counter-rotating screws. The volumes between the intermeshing screws and a liner or casing transport a specific volume of fluid. As the screws rotate the fluid volumes are transported from an inlet to an outlet of the pump.

Twin screw pumps are increasingly being used to aid in the extraction of oil and gas from on-shore and sub-sea wells. Twin screw pumps lower the back pressure on the reservoir and thereby enable greater total recovery from the reservoir.

Conventional twin screw pumps utilize shaft seals to prevent the flow of process fluids into the bearings, timing gears, motor, environment, or the like. In particular, twin screw pumps typically utilize a shaft seal on each end of each rotor, thereby requiring four seals in total. The usage of shaft seals is problematic for several reasons. The shaft seals are prone to failure. The shaft seals also typically require the usage of a lubricant flush system that maintains the rub surfaces of the sealing system clean and removes heat from the sealing surfaces.

It would be desirable to reduce in number or altogether eliminate shaft seals from a twin screw pump by eliminating the need for the required timing gears, and bearings to operate in a clean, controlled environment.

BRIEF DESCRIPTION

In accordance with one exemplary embodiment disclosed herein, a twin screw pump configured to transfer a process fluid is provided. The pump includes a pair of rotors disposed inside a casing having an inlet and an outlet. Each rotor includes a set of threads disposed on a portion of an outer surface of a shaft. A first bearing is coupled at a first end of the shaft and a second bearing is coupled at a second end of the shaft. The first and second bearings are not separated from the process fluid by one or more seals. The first and second bearings are configured for being lubricated by the process fluid when the twin screw pump is in an operational mode.

In accordance with one exemplary embodiment disclosed herein, a twin screw pump configured to transfer a multi-phase process fluid medium having a liquid medium and a gaseous medium is provided. The pump includes a pair of rotors disposed inside a casing having an inlet and an outlet. Each rotor includes a set of threads disposed on a portion of an outer surface of a shaft. A first bearing is coupled at a first end of the shaft and a second bearing is coupled at a second end of the shaft. The first and second bearings are not separated from the process fluid by one or more seals. A separating unit is coupled to the pump outlet and configured to receive the multi-phase fluid medium and separate the liquid medium from the gaseous medium. The separating unit is configured to maintain a predetermined quantity of liquid medium as a reserve quantity. The first and second bearings are configured for being lubricated by the liquid medium from the separating unit as required for operation.

In accordance with another exemplary embodiment disclosed herein, a system configured to transfer a process fluid medium is provided. The system includes a pump coupled to a motor and configured to be driven by the motor. The pump includes a pair of rotors disposed inside a casing having an inlet and an outlet. Each rotor includes a set of threads disposed on a portion of an outer surface of a shaft. A first bearing is coupled at a first end of the shaft, and a second bearing is coupled at a second end of the shaft. A single seal is coupled to a shaft of one of the rotors. The seal is configured between the motor and the pump to allow the motor to utilize a clean fluid for its purposes without contamination from the process fluids. This allows for a more conventional motor design and modular implementation.

In accordance with another exemplary embodiment disclosed herein, a twin screw pump configured to transfer a process fluid medium is provided. The pump includes a pair of rotors disposed inside a casing having an inlet and an outlet. Each rotor includes a set of threads disposed on a portion of an outer surface of a shaft. A first bearing is coupled at a first end of the shaft, and a second bearing is coupled at a second end of the shaft. A single seal is coupled to a shaft of one of the rotors. The seal is configured between the motor and the pump to allow the motor to utilize a clean fluid for its purposes without contamination from the process fluids. This allows for a more conventional motor design and modular implementation.

DRAWINGS

These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:

FIG. 1 is a diagrammatical representation of a twin screw pump in accordance with an exemplary embodiment disclosed herein;

FIG. 2 is a diagrammatical representation of a twin screw pump coupled to a motor and having a single shaft seal provided between the pump and the motor in accordance with an exemplary embodiment disclosed herein;

FIG. 3 is a diagrammatical representation of a conventional twin screw pump having a plurality of shaft seals provided between bearings;

FIG. 4 is a diagrammatical representation of a twin screw pump devoid of a plurality of seals provided between bearings in accordance with an exemplary embodiment disclosed herein; and

FIG. 5 is a diagrammatical representation of a journal bearing provided to a shaft of a twin screw pump in accordance with an exemplary embodiment disclosed herein; and

FIG. 6 is a diagrammatical representation of a fluid bearing provided to a shaft of a twin screw pump in accordance with an exemplary embodiment disclosed herein.

DETAILED DESCRIPTION

As discussed in detail below, exemplary embodiments disclosed herein provide a twin screw pump configured to transfer a process fluid, having a pair of rotors placed inside a casing. A pair of bearings is coupled respectively to both ends of the shaft of each rotor. The pair of bearings is not separated from the process fluids by seals and is lubricated by the process fluid medium. In one exemplary embodiment, a system configured to transfer a liquid medium is provided. The system includes a pump coupled to a motor and configured to be driven by the motor. A single seal is coupled to a shaft of one of the rotors of the pump and is located at a predetermined point between the motor and pump. The seal is configured between the motor and the pump to allow the motor to utilize a clean fluid for its purposes without contamination from the process fluids. In another exemplary embodiment, the shaft bearings of the twin screw pump includes ceramic or ceramic matrix composite bearings. In yet another exemplary embodiment, the pump configured to transfer a multi-phase fluid medium is provided. In such a system, the seal may include a barrier fluid sealing system or other types of sealing system configured to separate the multi-phase fluid medium into a liquid medium and a gaseous medium. The bearings of the pump are lubricated using the liquid medium. The exemplary twin screw pump provides better reliability, reduced rotor span, and enhanced pressure boost capability.

Referring to FIG. 1, a vertical twin screw pump 10 is illustrated in accordance with an exemplary embodiment disclosed herein. One end 12 of the pump 10 is coupled to a motor 18. The pump includes a pair of rotors 14, 16 having shafts 38,40 respectively arranged inside a pump chamber 24 enclosed by a casing 26 co-axially provided inside a pump casing 28. The pair of rotor shafts 38, 40 functions as delivery elements inside the casing 28. The motor 18 is configured to drive the pair of rotor shafts 38, 40. The rotor shaft 38 includes helical screws 30, 34 which interlocking with helical screws 32, 36 of the adjoining rotor 40. A center section 46 of the pump chamber 24 is communicatively coupled via outlets 48, 50 provided in the pump casing 28 to a delivery chamber 52 having a delivery opening 54 provided in the pump casing 28. The delivery chamber 52 formed between the pump casing 28 and a peripheral wall of the pump chamber 24 includes an upper chamber 56 communicatively coupled to an upper feeding opening 58 and a lower chamber 60 communicatively coupled to a lower feeding opening 62 of the pump. Fluid inlets 64, 66 are provided in the pump casing 28. It should be noted herein that the feed direction of the process fluid medium depends on the direction of rotation of the shafts 38, 40. The flow direction may be advantageously reversed in some exemplary embodiments by reversing the rotation direction of rotor shafts 38, 40.

In the embodiment shown in FIG. 1, a fluid medium that enters the pump 10 through the inlets 64, 66 is split in two partial streams to the ends of the pump chamber 24 and is conveyed to the center section 46 of the pump and is discharged out through the opening 54. In another embodiment the inlet, outlet, and direction of flow are reversed by rotating the screws in the opposite direction.

Referring to FIG. 2, a twin screw pump 10 coupled to the motor 18 is illustrated in accordance with an exemplary embodiment disclosed herein. In the illustrated embodiment the pump 10 and the motor 18 are configured along a horizontal direction. The pump casing 28 has fluid inlets 64, 66 and a delivery opening 54. In the illustrated embodiment, a pair of support bearings, shown as first bearing 68 and a second bearing 70 are coupled respectively to a first end 72 and a second end 74 respectively of the shaft 38 i.e. driven shaft. Also, another pair of bearings shown as first bearing 76 and second bearing 78, are coupled respectively to a first end 80 and a second end 82 of the shaft 40 i.e. drive shaft. The bearing housing may be an integrated radial/thrust bearing rather than an independent bearing housing. A motor shaft 84 of the motor 18 is coupled via a coupling 86 to the drive shaft 40 of the pump 10. Timing gears 88 are coupled to the shafts 38, 40 and located proximate to the drive end of the pump 10. In certain other embodiments, timing gears may be provided to the other end of the pump and may be located inboard or outboard of the bearings as preferred. Conventional twin screw pumps have shaft seals provided on each rotor that separate the flow of the process fluid medium from the bearings and timing gears which operate within a clean barrier fluid. However, conventional sealing system may not provide a true barrier against the flow of process fluid medium because smaller sand particles may navigate past the shaft seals to the bearings, timing gears, and pump casing with detrimental consequences.

In illustrated exemplary embodiment, the bearings 68, 70, 76, and 78 include ceramic bearings, ceramic matrix composite (CMC) bearings or ceramic particles in a metallic matrix (cemented tungsten carbide). Ceramic, CMC and cemented tungsten carbide bearings are resistant to damage from foreign particles (particularly sand) on account of their high hardness. The exemplary bearings may not be separated from the process fluids by seals while nonetheless providing reliable operation. Since the seals are eliminated in the exemplary screw pump, the distance between the bearings may be reduced. As a result, the rotor span may also be reduced resulting in a stiffer and more compact system. In the absence of seals, the bearings may require lubrication by the liquid component of the process medium, for example water and/or oil.

In the illustrated embodiment, a single seal 90 is coupled to the drive shaft 40 of the pump 10 and is located at a predetermined point or location 92 between the pump 10 and the motor 18. The seal 90 is configured to obstruct process fluids from entering the motor 18. A conventional twin screw pump has two shaft seals provided on the ends of each rotor shaft (that is, a total of at least four seals) for separating the process fluid flow from the bearings, timing gears, and casing. The illustrated exemplary twin screw pump operates with a single shaft seal (meaning only one shaft seal is present and is situated on one of the two rotors) instead of a plurality of shaft seals. By reducing the portion of the rotor occupied by the plurality of seals, the rotor span of the illustrated exemplary screw pump may also be reduced. Therefore, the exemplary screw pump is relatively compact.

Referring to FIG. 3, a conventional twin screw pump 94 is illustrated. The pump 94 has a housing 95 provided with fluid inlets 97, 99 and a delivery opening 109. The illustrated pump includes a first bearing 96 and a second bearing 98 coupled respectively to a first end 100 and a second end 102 respectively of the drive shaft 104. Also, a first bearing 106, and a second bearing 108 are coupled respectively to a first end 110 and a second end 112 of the driven shaft 114. The drive shaft 104 and the driven shaft 114 are coupled respectively to the timing gears 116. The timing gears 116 may be similarly located on the other end of the rotor shafts 104 and 114, and may be located inboard or outboard of the bearings 96 and 106 or 98 and 108 as preferred. The timing gears may include cemented tungsten carbide, ceramic matrix composite (for example, silicon carbide composite), monolithic ceramic (for example, silicon carbide), or the like. One pair of shaft seals 118, 120 are coupled to the drive shaft 104 and provided proximate to the bearings 96, 98 respectively. Another pair of shaft seals 122, 124 are coupled to the driven shaft 114 and provided proximate to the bearings 106, 108 respectively. The shaft seals 118, 120, 122, 124 are configured not to isolate the bearings and timing gears from the process fluid but to maintain a liquid portion of the process fluid about the bearings and timing gears as required for their operation while minimizing pumping efficiency losses. These shaft seals controls flow through the bearings and timing gears to limit rise in temperature. As such these shaft seals are not as critical as isolation seals and may be of more robust design. The drive shaft 104 is coupled via a coupling 126 to a shaft 128 of a motor 130.

Referring to FIG. 4, a twin screw pump 10 is illustrated in accordance with an exemplary embodiment disclosed herein. The exemplary twin screw pump 10 includes the first bearing 68 and the second bearing 70 coupled respectively to the first end 72 and the second end 74 respectively of the driven shaft 38. Also, the first bearing 76, and the second bearing 78 are coupled respectively to a first end 80 and a second end 82 of the drive shaft 40. As discussed above, in one embodiment, the illustrated bearings include ceramic, CMC or cemented tungsten carbide. The drive shaft 40 and the driven shaft 38 are coupled respectively to the timing gears 88. In the illustrated embodiment, no seals are provided between the bearings provided on the driven and drive shafts 38, 40.

In the illustrated embodiment, the pump 10 is configured to transfer a multi-phase fluid. The multi-phase fluid may include a liquid medium and a gaseous medium. The multi-phase fluid enters the pump casing 28 via the inlets 64, 66 at an inlet pressure (Ps). The multi-phase fluid is discharged via the delivery opening 54 of the casing 28 at a discharge pressure (Pd). The discharged fluid is fed to a separating unit 132 configured to separate the multi-phase fluid into the liquid medium and the gaseous medium. Any gas/liquid separator known to those skilled in the art may be used in combination with the exemplary pump. In the illustrated embodiment, the separating unit 132 is designed to maintain a predetermined quantity of liquid medium as a reserve against periods of time in which the pumped medium may include an otherwise insufficient amount of liquid content. The reserve quantity of liquid medium facilitates to compensate for an operating condition in which an amount of liquid medium in the multi-phase fluid medium is less than a predetermined quantity. The gaseous medium and excess liquid medium are discharged at a boosted pressure from the separating unit 132 into a downstream pipeline. The liquid medium is fed at the pump discharge pressure (Pd) into the casing 28 via one or more flow control orifices or auxiliary pump 134. In certain exemplary embodiments, both flow control orifices and auxiliary pumps may be used in parallel to control the flow of liquid medium from the separating unit 132 to the casing 28. The orifices may be a fixed orifice or a variable orifice depending upon the requirement. The orifices/pump 134 are configured to reduce the pressure of the liquid medium flow from the discharge pressure (Pd) to a lower pressure (Ps+ΔP), where ΔP is the pressure differential between the suction pressure (Ps) and discharge pressure (Pd). The liquid medium is fed at pressure (Ps+ΔP) into the casing 28. In other words, when there is sufficient pressure drop, the orifices can maintain sufficient flow of liquid from the separator 132 to the pump cavity. When there is insufficient pressure drop, for example during startup, auxiliary pump may be used in such a way that sufficient flow of liquid medium is ensured from the separator 132 to the pump cavity. In this manner it is ensured that the bearings/timing gears fluid requirements are satisfied. In certain exemplary embodiments, liquid medium is passed in heat exchange relationship with surrounding coolant water via a heat exchanger (not shown). The liquid medium may be cooled to a substantially lower temperature before being fed into the casing 28. The cooling of liquid medium may be done to increase the viscosity and heat capacity of the liquid medium prior to delivery to the bearings and timing gears. The bearings and timing gears are lubricated by the liquid medium when the twin screw pump is in an operational mode. It should be noted herein that operational mode may be referred to as start up condition or normal operating condition of the pump. In certain other embodiments, the separator 132 may be provided to the upstream side of the pump. In certain other exemplary embodiments, if other auxiliary liquid sources are available, then the bearings may be lubricated using the liquid medium from the auxiliary liquid sources. In certain other exemplary embodiments fluid may be actively pumped into the pump cavity before the pump starts rotating.

Referring to FIG. 5, a bearing 76 of the drive shaft 40 is illustrated in accordance with an exemplary embodiment disclosed herein. In the illustrated embodiment, the bearing 76 includes a journal bearing. The bearing 76 includes a sleeve or a cylinder 136 configured surrounding a journal of the shaft 40. A layer of lubricant 138 separates the bearing sleeve 136 from the shaft journal. In the illustrated embodiment, fluid stream itself is used as a lubricant for the bearing 76. For example, oil, water, or the like may be used as the lubricant.

The shaft rotation creates a fluid wedge that supports the shaft and relocates it within the bearing clearances. The exemplary stable bearing design holds the shaft 40 at a fixed attitude angle during transient periods such as machine startups/shutdowns or load changes. The damping properties of the fluid lubricant also cause the lubricant to act as an excellent medium for limiting vibration transmission. Since the bulk modulus of the liquid stream is relatively higher than for gas, the load capacity of the bearing is enhanced. As a result, lower speed operation of the pump is enabled. Moreover, lubrication of a higher weight rotor is also enabled. In the exemplary embodiment, a separate lubrication system is not required for the bearing. In one exemplary embodiment, the sleeve may include cemented tungsten carbide, ceramic matrix composite (for example, silicon carbide composite), monolithic ceramic (for example, silicon carbide), or the like. In certain other exemplary embodiments, the journal bearings may include materials such as white metal, babbit metal, phosphor bronze, or combinations thereof. Even though only one bearing is illustrated, other bearings of the drive and driven shaft may also include journal bearings.

Referring to FIG. 6, a bearing 68 of the driven shaft 38 is illustrated in accordance with an exemplary embodiment disclosed herein. In the illustrated embodiment, the bearing 68 may include a high-load capacity rigid tilt-pad type bearing. Tilt-pads may include cemented tungsten carbide, ceramic matrix composite (for example, silicon carbide composite), monolithic ceramic (for example, silicon carbide), or the like. It should be noted herein that list of materials for sleeves, tilt pads, and is not all-inclusive and other suitable materials are also envisaged. The illustrated exemplary bearing 68 includes a plurality of partitioned bearing pads 140 provided within a bearing outer rim 146 and located proximate an outer surface of the shaft 38. The bearing pads 140 may include monolithic sintered ceramic pads, ceramic matrix composite pads, silicon carbide pads, cemented carbide pads or the like. In one example, each bearing pad 140 is coupled via a biasing spring 142 to an inner surface of the bearing outer rim 146. The shaft 38 and the bearing pads 140 are separated by the fluid's high pressure, which is generated by the rotation of the shaft, which pulls the fluid stream into the bearing 68 via viscosity effects. In the illustrated embodiment, fluid stream itself is used as a lubricant for the bearing 68. Other bearings of the drive and driven shaft may also employed. As mentioned above, it is advantageous to provide liquid to the bearing as opposed to gas, on account of the higher bulk modulus of the liquid.

While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention. 

1. A twin screw pump configured to transfer a process fluid having a liquid medium and a gaseous medium, the pump comprising: a casing comprising an inlet and an outlet; and a pair of rotors disposed inside the casing, each rotor comprising a shaft, a set of threads disposed on a portion of an outer surface of the shaft, a first bearing coupled at a first end of the shaft; and a second bearing coupled at a second end of the shaft, wherein the first and second bearings are not separated from each other by a seal, wherein the first and second bearings are configured for being lubricated by the process fluid when the twin screw pump is in an operational mode.
 2. The pump of claim 1, wherein the first and second bearings comprise journal bearings.
 3. The pump of claim 1, wherein the first and second bearings comprise tilt pads comprising cemented tungsten carbide, ceramic matrix composite, or monolithic ceramic.
 4. The pump of claim 1, wherein the first and second bearings comprise sleeves comprising cemented tungsten carbide, ceramic matrix composite, or monolithic ceramic.
 5. The pump of claim 1, wherein the first and second bearings are located within the casing.
 6. The pump of claim 1, wherein the liquid medium comprises water.
 7. The pump of claim 1, wherein the liquid medium comprises oil.
 8. A twin screw pump configured to transfer a multi-phase fluid medium having a liquid medium and a gaseous medium, the pump comprising: a casing comprising an inlet and an outlet; a pair of rotors disposed inside the casing, each rotor comprising a shaft, a set of threads disposed on a portion of an outer surface of the shaft, a first bearing coupled at a first end of the shaft; and a second bearing coupled at a second end of the shaft, wherein the first and second bearings are not separated by a seal, and a separating unit configured to receive the multi-phase fluid medium and separate the liquid medium from the gaseous medium; wherein the first and second bearings are configured for being lubricated by the liquid medium when the twin screw pump is in an operational mode.
 9. The pump of claim 8, wherein the separating unit is configured to store a predetermined reserve quantity of liquid medium to compensate for an operating condition in which an amount of liquid medium in the multi-phase fluid medium is less than a predetermined quantity.
 10. The pump of claim 9, wherein the separating unit is configured to discharge the gaseous medium and excess liquid medium at a boosted pressure from the separating unit.
 11. The pump of claim 8, further comprising one or more flow control orifices, and/or auxiliary pumps; wherein the separating unit is configured to transfer the liquid medium to the casing via the flow control orifices and/or auxiliary pumps.
 12. The pump of claim 8, further comprising one or more timing gears coupled to the shaft of each rotor; wherein the one or more timing gears are configured for being lubricated by the liquid medium when the twin screw pump is in an operational mode.
 13. The pump of claim 12, wherein the timing gears comprises cemented tungsten carbide, ceramic matrix composite, or monolithic ceramic.
 14. A system configured to transfer a process fluid, the system comprising: a motor; a pump coupled to the motor and configured to be driven by the motor, the pump comprising a casing comprising an inlet and an outlet; and a pair of rotors disposed inside the casing; each rotor comprising a shaft, a set of threads disposed on a portion of an outer surface of the shaft, a first bearing coupled at a first end of the shaft, a second bearing coupled at a second end of the shaft, and a single seal coupled to a shaft of one of the rotor, wherein the seal is configured to prevent the flow of process fluid into the motor.
 15. The pump of claim 14, wherein the first and second bearings comprise sleeves comprising cemented tungsten carbide, ceramic matrix composite, or monolithic ceramic.
 16. The pump of claim 14, wherein the first and second bearings comprise tilt pads comprising cemented tungsten carbide, ceramic matrix composite, or monolithic ceramic.
 17. The pump of claim 14, wherein the first and second bearings are located within the casing.
 18. A twin screw pump configured to transfer a process fluid, the pump comprising: a casing comprising an inlet and an outlet; and a pair of rotors disposed inside the casing; each rotor comprising a shaft; a set of threads disposed on a portion of an outer surface of the shaft; a first bearing coupled at a first end of the shaft; a second bearing coupled at a second end of shaft, wherein the first and second bearings comprises ceramic composite bearings; and a single seal coupled to a shaft of one of the rotors, wherein the seal is configured to prevent the flow of process fluid into the motor.
 19. The pump of claim 18, wherein the first and second bearings comprise sleeves comprising cemented tungsten carbide, ceramic matrix composite, or monolithic ceramic.
 20. The pump of claim 18, wherein the first and second bearings comprise tilt pads comprising cemented tungsten carbide, ceramic matrix composite, or monolithic ceramic.
 21. The pump of claim 18, wherein the first and the second bearings are located within the casing. 